In recent years, the polysilicon TFT-LCD (Thin Film Transistor Liquid Crystal Display) is drawing people's attention because it is capable of displaying images with higher luminance than the TFT-LCD that uses amorphous silicon films. In the production of a polysilicon TFT-LCD, a glass plate is coated with a polysilicon film to create a polysilicon substrate. Then, the face of the polysilicon substrate is divided into a large number of picture elements arrayed in a two-dimensional pattern, and a thin film transistor is formed on each picture element to obtain an LCD substrate. To produce a polysilicon TFT-LCD having a large display area, it is necessary to create a high-quality polysilicon substrate that particularly has a high degree of evenness.
Polysilicon substrates are also gathering attention as a high-performance solar cell substrate. With the growing demand for and application of solar cells, it is necessary for the polysilicon substrate to have a larger area. Apart from that, ordinary substrates for semiconductor devices also need to be produced by a deposition process if their area is larger than a single crystal.
The production of substrates used in the aforementioned fields requires a treatment that uses plasma. This treatment includes the steps of depositing a material for the substrate onto a base plate to be processed and etching the surface of the same plate. Larger substrates require a greated degree of a plasma treatment system. The most serious problem for a large-scale system is the unevenness in the plasma treatment. To avoid this problem, the plasma density should be as uniform as possible over the entire area of the substrate. From the viewpoint of productivity, it is necessary to increase the plasma density so as to improve the deposition speed or the etching rate.
Examples of the methods of generating plasma include the ECR (electron cyclotron resonance) plasma, microwave plasma, the inductively coupled plasma and the capacity coupled plasma. In the inductively coupled plasma method, a radiofrequency (RF) voltage is applied to an induction coil serving as an antenna to create an induction electromagnetic field within the plasma generator, whereby plasma is produced. This method satisfies one of the aforementioned requirements: the generation of high-density plasma. With regard to the other requirement concerning the improvement in the uniformity of the plasma density, a variety of antennas differing in form, position and other factors have been proposed, taking into account the dependency of the plasma density on the distance from the antenna. For example, the Japanese Unexamined Patent Publication No. 2000-58297, which is referred to as the “Patent Document 1” hereinafter, discloses a technique in which the uniformity of the plasma intensity is improved by introducing an RF wave through a flat coil located on the outside of the ceiling of the plasma-generating chamber.
To produce a large size substrate using the above-described construction, it is necessary to adequately increase the wall thickness of the ceiling in the plasma-generating chamber to provide the ceiling with a sufficient mechanical strength. However, this makes it difficult for the system of Patent Document 1 to obtain an adequate strength of electromagnetic field within the plasma-generating chamber, because the antenna is located outside the plasma-generating chamber and the thick wall attenuates the induction electromagnetic field radiated from the antenna. In summary, the method disclosed in Patent Document 1 is effective in enhancing the uniformity of the plasma density to a certain extent but cannot adequately increase the plasma density.
To solve this problem, the inventors of the present patent application have proposed that the RF antenna should be located within the plasma-generating chamber, multiple antennas should be used, and each antenna should have a non-loop shape (i.e. a shape that does not completely surround a space), as disclosed in the Japanese Unexamined Patent Publication No. 2001-35697 (“Patent Document 2”).
This construction enables the electromagnetic field to be fully radiated within the plasma-generating chamber without being attenuated by the wall of the chamber, so that the plasma density is adequately increased. Also, the equally spaced multiple antennas create a highly uniform radiation of electromagnetic field, which improves the uniformity of the plasma intensity. The use of multiple antennas makes the inductance of each antenna small enough to prevent an abnormal discharge, which often takes place if the antenna is located within the chamber and a high voltage is applied to the antenna. The non-loop shape of the antenna also reduces the inductance of the antenna and accordingly contributes to the suppression of the abnormal discharge. With these effects obtained, it is now possible to carry out a deposition process or etching process on a base plate of a large area. In the following description, the construction using multiple antennas as disclosed in Patent Document 2 is referred to as the “multi-antenna system.”
To process a much larger substrate in the future, it is necessary to generate a plasma state with a higher degree of uniformity while ensuring an adequate level of plasma density. For that purpose, the multi-antenna system needs to be further examined with respect to the shape, position and other factors of each antenna, the relationship between the antennas, and other parameters that have not been considered. Also, it is possible that the electromagnetic field radiated from the antenna forms a standing wave, which deteriorates the uniformity of the plasma. Furthermore, the multi-antenna system still allows the plasma density to be lower at the central region of the substrate than at the marginal region because the strength of the electromagnetic field depends on the distance from the RF antenna. If the area of the substrate is small, the difference in plasma density between the central region and the marginal region of the substrate will be kept within the allowable range. However, the distance cannot be ignored for large substrates. Finally, the ion species or the radical species to be created must be also considered because the etching rate or the deposition speed varies depending on the ion species or the radical species.
To address the aforementioned problems, the present invention aims to provide a plasma generator capable of creating a spatially uniform distribution of high-density plasma and controlling the type of ion species or the radical species to be created.